Fabrication method for heterojunction bipolar transistor

ABSTRACT

A fabrication method for heterojunction bipolar transistor is disclosed. The method uses ISSG oxide instead of conventional PECVD oxide so that the base/emitter interface damage can be reduced. Moreover, the invention replaces the conventional emitter-window/space mask with an emitter-window reverse-tone mask/line mask to minimize the critical dimension of emitter window. Furthermore, the invention also utilizes a two-steps extrinsic base implantation to form two extrinsic bases with different dopant concentrations so that the base resistance can be reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fabrication method for heterojunctionbipolar transistor (HBT), and more particularly to a fabrication methodfor heterojunction bipolar transistor (HBT) emitter/emitter window by areverse-tone approach.

2. Description of the Related Art

Transistors are multi-electrode semiconductor devices in which thecurrent flowing between two specified electrodes is controlled ormodulated by the voltage applied at a third (control) electrode.Transistors fall into two major classes: the bipolar junction transistor(BJT) and the field-effect transistor (FET). BJTs were derived from thepoint-contact transistor, which was invented at Bell TelephoneLaboratories in 1947 by Bardeen, Brattain, and Shockley. BJTs comprisetwo p-n junctions placed back-to-back in close proximity to each other,with one of the regions common to both junctions. This forms either ap-n-p or n-p-n transistor comprising three regions—emitter, base andcollector. The BJT utilizes the flow of both electrons and holes acrossthe p-n junctions for its electrical behavior. That is, the current flowthrough the emitter and collector electrodes is controlled by thevoltage across the base-emitter p-n junction.

In normal (or forward active) operation of a BJT, the base-emitter p-njunction is forward biased and the base-collector junction is reversebiased. Majority-carrier current flows across the forward-biasedemitter-base junction. The emitter is much more heavily doped than thebase region, so that most of the total current flow across thebase-emitter junction consists of majority carriers from the emitterinjected into the base. These injected carriers become minority carriersin the base region, and will tend to recombine. Such recombination isminimized by making the base region very narrow, so that the injectedcarriers can diffuse across the base to the reverse-biasedbase-collector junction, where they are swept across the junction intothe collector, to appear in the outside circuit as the collectorcurrent. The magnitude of this collector current depends on the numberof majority carriers injected into the base from the emitter, and thuscurrent is controlled by the base-emitter p-n junction voltage. Theoutput (collector) current is therefore controlled by the input(base-emitter) voltage, and the output circuit of the transistor can bemodeled as a voltage-controlled current source (dependent sources),while the input circuit looks like a p-n junction diode.

In principle, the transistor can be operated in reverse active mode byreversing the connections. However, in practice, the transistor is notcompletely symmetrical. That is, the emitter is very heavily doped tomaximize emitter injection, and the collector is relatively lightlydoped so that it can accommodate large voltage swings across itsreverse-biased junction. While the electrical characteristics aresimilar in appearance, the forward characteristics show much greatergain, as expected.

If both junctions are reverse biased, the transistor behaves like anopen switch, with only the p-n junction reverse leakage currentsflowing. If both junctions are forward biased, there is injection ofcarriers into the base region from both sides, and a low resistance ispresented to current flow in either direction: the transistor behaveslike a closed switch, and the base stores the injected charge.

BJTs can be used to provide linear voltage and current amplification:small variations of the base-emitter voltage and hence the base currentat the input terminal result in large variations of the output collectorcurrent. Since the transistor output has the appearance of a currentsource, the collector can drive a load resistance and develop an outputvoltage across this resistance (within the limits of the supplyvoltage). The transistor can also be used as a switch in digital logicand power switching applications, switching from a high-impedance offstate in cut-off, to a low-impedance on state in saturation. Inpractice, full saturation conditions of base-collector forward biasedare generally avoided, to limit the carrier storage in the base andreduce the switching time. Such BJTs find application in analog anddigital circuits and integrated circuits, at all frequencies from audioto radio frequency. For higher frequencies, such as microwaveapplications, heterojunction bipolar transistors (HBTs) are used.

HBTs are bipolar junction transistor which incorporate a wide band gapemitter, where the emitter-base junction is a heterojunction betweensemiconductors of different energy band gaps. The following are typicalmaterials for HBTs: aluminum-gallium-arsenide(AlGaAs)(emitter)/gallium-arsenide(GaAs)(base); aluminum-indium-arsenide(AlInAs)/indiumgallium-arsenide (InGaAs); Si/silicon-germanium (SiGe);and indium-gallium-phosphide (InGaP)/GaAs; indium-phosphide(InP)/InGaAs.

However, conventional HBTs and the forming process thereof have severaldrawbacks. FIG. 1A to FIG. 1C show a conventional HBT process. Referringto FIG. 1A, a substrate 102 having a collector, a dielectric 104, alayer 106, a base layer 108, a dielectric layer 110, a hard mask layer112, a BARC layer 114 and a photoresist layer 116 with a window patternis shown. Then the BARC layer 114, the hard mask layer 112 and thedielectric layer 110 are etched by reactive ion etching (RIE) and wetdip processes to expose the base layer 108 as shown in FIG. 1B. Finally,an emitter 118 is formed and an extrinsic base region 120 is formed byan ion implantation process.

The above-mentioned process has several drawbacks. Firstly, the criticaldimension control of the emitter window is hard and tough because theemitter window pattern is formed by a photo mask with a window/holepattern which is hardly shrink, especially when the critical dimensioncontrol shrinks toward to 0.18 micron generation. Moreover, the emitterwindow pattern formed by the window/hole pattern tends to enlarge in aphotolithography process. Furthermore, the emitter window formed byreactive ion etching and wet dipping the BARC layer 114, the hard masklayer 112 and the dielectric layer 110 will be further enlarged. Defectssuch as voids induced by RIE and wet dip will be formed between theemitter 118 and the base layer 108 after the emitter 118 is formed onthe base layer 108.

Thus it is necessary to provide a new method to resolve the drawbacksset forth. It is towards those goals that the present invention isspecifically directed.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a fabricationmethod for heterojunction bipolar transistor (HBT) which can reducebase/emitter interface damages.

It is another object of this invention to provide a fabrication methodfor heterojunction bipolar transistor (HBT) which utilizes anemitter-window reverse-tone mask/line mask to minimize the criticaldimension of emitter window.

It is a further object of this invention to provide a fabrication methodfor heterojunction bipolar transistor (HBT) with a two-steps extrinsicbase implantation to reduce base resistance.

To achieve these objects, and in accordance with the purpose of theinvention, the invention uses a fabrication method for formingheterojunction bipolar transistor. The method comprises the followingsteps. First of all, a substrate having a collector therein, a baselayer over the substrate, an oxide layer over the base layer, and apolysilicon layer over the oxide layer is provided. Then a firstphotoresist layer is formed over the polysilicon layer. Next a linepattern is transferred into the first photoresist layer by a photo maskwith the line pattern to form an emitter window pattern. Then saidpolysilicon layer is etched to expose said oxide layer by using saidemitter window pattern as an etching mask. Next a first ion implantationprocess is performed into said base layer to form a first extrinsic baseregion therein. Then said emitter window pattern is removed and adielectric layer is formed over the polysilicon layer and the oxidelayer. Next the dielectric layer is thinned to expose the polysiliconlayer. Then the polysilicon layer is removed to expose the oxide layer.Next the exposed oxide layer is etched to expose the base layer. Then anemitter layer is formed over the base layer and the dielectric layer.Next a second photoresist layer is formed over the emitter layer. Thenan emitter pattern is transferred into the second photoresist layer.Next the emitter layer is etched to form an emitter and expose the oxidelayer by using the emitter pattern as an etching mask. Finally, a secondion implantation process is performed to form a second extrinsic baseregion.

In another embodiment of this invention, the invention uses anotherfabrication method for forming heterojunction bipolar transistor. Themethod comprises the following steps. First of all, a substrate having acollector therein, a base layer over the substrate, a first emitterlayer over the base layer, and a first dielectric layer over the firstemitter layer is provided. Then a first photoresist layer is formed overthe first dielectric layer. Next a line pattern is transferred into thefirst photoresist layer by a photo mask with the line pattern to form aemitter window pattern. Then the first dielectric layer is etched toexpose the first emitter layer by using the emitter window pattern as anetching mask. Next the emitter window pattern is removed and the firstemitter layer is etched to expose the base layer. Then a first oxidelayer is formed over the base layer and the first emitter layer. Next asecond oxide is formed over the first dielectric layer and the firstoxide layer. Then a first ion implantation process is performed into thebase layer to form a first extrinsic base region therein. Next a seconddielectric layer is formed over the second oxide layer. Then the seconddielectric layer is anisotropically etching to expose the second oxidelayer and form a spacer. Next a second ion implantation process isperformed to form a second extrinsic base region. Then a thirddielectric layer is formed over the second oxide layer and the spacer.Next the third dielectric layer is thinned to expose the second oxidelayer. Then the exposed second oxide layer and the first dielectriclayer are removed to expose the first emitter layer. Next a secondemitter layer is formed over the first emitter layer and the thirddielectric layer. Then a second photoresist layer is formed over thesecond emitter layer. Next an emitter pattern is transferred into thesecond photoresist layer and the second emitter layer, the thirddielectric layer, the second oxide layer and the first oxide layer areetched to form an emitter and expose the second extrinsic base region.

The invention provides a heterojunction bipolar transistor comprising asubstrate having a collector therein, an intrinsic base region, a firstextrinsic base region, a second extrinsic base region, an emitter on theintrinsic base layer and a spacer adjacent the emitter and on the firstextrinsic base region. The first extrinsic base region is adjacent theintrinsic base region and the second extrinsic base region is adjacentthe first extrinsic base region on the substrate, wherein a dopantconcentration of the second extrinsic base region is higher than adopant concentration of the first extrinsic base region.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1A to FIG. 1C show a process flow of a conventional HBT process;

FIG. 2A to FIG. 2H show process steps of one embodiment this invention;and

FIG. 3A to 3H show process steps of another embodiment this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It is to be understood and appreciated that the process steps ndstructures described below do not cover a complete process flow andstructure. The present invention can be practiced in conjunction withvarious fabrication techniques that are used in the art, and only somuch of the commonly practiced process steps are included herein as arenecessary to provide an understanding of the present invention.

The present invention will be described in detail with reference to theaccompanying drawings. It should be noted that the drawings are ingreatly simplified form and they are not drawn to scale. Moreover,dimensions have been exaggerated in order to provide a clearillustration and understanding of the present invention.

Referring to FIG. 2A, a substrate 202 having a collector therein isfirstly provided. The substrate 202 comprises a p-type epitaxial siliconsubstrate. Then a dielectric layer 204 and a layer 206 are sequentiallyformed on the substrate 202. The layer 206 comprises a polysiliconlayer. Next a window is formed to expose the substrate 202 by patterningand etching the dielectric layer 204 and the layer 206. Then a baselayer 208 is formed over the layer 206 and to fill the window. The baselayer 208 comprises a silicon germanium (SiGe) layer. Next an oxidelayer 210 is formed over the base layer 208. The oxide layer 210comprises an in situ steam generation (ISSG) oxide layer grew on a SiGelayer. The oxide layer 210 is used as a gate oxide layer. Then a layer212 is formed on the oxide layer 210. The layer 212 comprises an undopedpolysilicon layer. Next a bottom anti-reflecting coating (BARC) layer214 is formed over the layer 212. Finally, a photoresist layer is formedon the BARC layer 214 and is patterned to form a line pattern 216. Theline pattern 216 is formed by using a reverse-tone mask. Thereverse-tone mask comprises photo masks used to expose photoresistlayers and form line patterns of circuit lead line of integratedcircuit. Since the line pattern 216 is a result of removing most of thephotoresist layer after a developing process and is used as an etchingmask, the line pattern 216 tends to decrease its size during exposingand developing processes. Moreover, a line pattern scale are much easierto be decreased than a window pattern scale or a hole pattern scalesince the line pattern covers and protects the device (such as a leadline) to be formed from the etchant while the window pattern or the holepattern expose the device (such as a contact window or a hole) to beformed to the etchant during etching, and the etching mechanism tends toremove more material than demand. Therefore, it is much easier todecrease the scale of emitter window by using a reverse-tone mask or aphoto mask for line pattern.

Referring to FIG. 2B, the BARC layer 214 and the layer 212 are etched toexpose the oxide layer 210 by using the line pattern 216 as an etchingmask. Then an ion implantation process is performed to form a firstextrinsic base region 218 in the base layer 208 by using the linepattern 216 as an implantation mask. FIG. 2C shows a result of strippingthe line pattern 216 and forming a dielectric layer 220 over the layer212 and the oxide layer 210. The dielectric layer 220 comprises a SiNlayer. The dielectric layer 220 is then planarized by a chemicalmechanical polishing process and is further etched back to expose thelayer 212 as shown in FIG. 2D. Then the layer 212 is removed to exposethe oxide layer 210 and the exposed oxide layer 210 is etched to exposethe base layer 208 as shown in FIG. 2E.

Referring to FIG. 2F, an emitter layer 222 is formed over the dielectriclayer 220 and the base layer 208. Then a photoresist layer is formed onthe emitter layer 222 and is patterned to form an emitter pattern 224.The emitter layer 222 and the dielectric layer 220 are etched to exposethe oxide layer 210 by using the emitter pattern 224 as an etching maskas shown in FIG. 2G. Then an ion implantation process is performed toform a second extrinsic base region 226, and the base layer comprises anintrinsic base region 208, the first extrinsic base region 218 and thesecond extrinsic base region 226. Finally, the emitter pattern 224 isremoved as shown in FIG. 2H.

The invention uses ISSG oxide instead of conventional PECVD oxide sothat the base/emitter interface damage can be reduced. Moreover, theinvention replaces the conventional emitter-window/space mask with anemitter-window reverse-tone mask/line mask to minimize the criticaldimension of emitter window. Furthermore, the invention also utilizes atwo-steps extrinsic base implantation to form two extrinsic bases withdifferent dopant concentrations so that the base resistance can bereduced.

In another embodiment of this invention, another fabrication method forheterojunction bipolar transistor (HBT) emitter/emitter window isprovided. First of all, a substrate 302 having a collector therein isprovided as shown in FIG. 3A. The substrate 302 comprises a p-typeepitaxial silicon substrate. Then a dielectric layer 304 and a layer 306are sequentially formed on the substrate 302. The layer 306 comprises apolysilicon layer. Next a window is formed to expose the substrate 302by patterning and etching the dielectric layer 304 and the layer 306.Then a base layer 308 is formed over the layer 306 and to fill thewindow. The base layer 308 comprises a silicon germanium (SiGe) layer.Then a clean process by using a HF solution can be performed on the baselayer 308. Next a first emitter layer 310 is formed over the base layer308. The first emitter layer 310 comprises an undoped polysilicon layer.Then a dielectric layer 312 used as a hard mask is formed over the firstemitter layer 310. The dielectric layer 312 comprises a TEOS (TetraEthyl Ortho Silicate Glass) layer. Next a bottom anti-reflecting coating(BARC) layer 314 is formed over the dielectric layer 312. Finally, aphotoresist layer is formed on the BARC layer 314 and is patterned aline pattern 316. The line pattern 316 is formed by using a reverse-tonemask. The reverse-tone mask comprises photo masks used to exposephotoresist layers and form line patterns of circuit lead line ofintegrated circuit.

Referring to FIG. 3B, the BARC layer 314, the dielectric layer 312 andthe first emitter layer 310 are etched to expose the base layer 308.Then the line pattern 316 and the BARC layer 314 are stripped. Next anoxide layer 318 is formed over the base layer 308 and the first emitterlayer 310. The oxide layer 318 comprises an in situ steam generation(ISSG) oxide layer grew on a SiGe layer and a polysilicon layer. Then alinear oxide layer 320 is formed over the oxide layer 318 and thedielectric layer 312. Finally, a first ion implantation process isperformed to form a first extrinsic base region 322.

Referring to FIG. 3C, a dielectric layer is formed over the linear oxidelayer 320 and is etched to form a spacer 324. Then a second ionimplantation process is performed to form a second extrinsic base region326. Then referring to FIG. 3D, a dielectric layer 328 is formed overthe linear oxide layer 320 and the spacer 324. The dielectric layer 328comprises a SiN layer. The dielectric layer 328 is then planarized by achemical mechanical polishing process and is etched back to expose thelinear oxide layer 320 as shown in FIG. 3E.

Referring to FIG. 3F, the exposed linear oxide layer 320 and thedielectric layer 312 are removed to expose the first emitter layer 310.Then a second emitter layer 330 is formed over the dielectric layers 324and 328 and the exposed first emitter layer 310. Next a photoresistlayer is formed on the second emitter layer 330 and is patterned to forma line pattern 332 as shown in FIG. 3G. The line pattern 332 is formedby using a reverse-tone mask. The reverse-tone mask comprises photomasks used to expose photoresist layers and form line patterns ofcircuit lead line of integrated circuit. Then the second emitter layer330, the dielectric layer 328, the linear oxide layer 320 and the oxidelayer 318 are etched by using the line pattern 332 as an etching mask toexpose the second extrinsic base region 326. Finally, a heterojunctionbipolar transistor of the invention is formed as shown in FIG. 3H. Theheterojunction bipolar transistor comprising a substrate having acollector therein, an intrinsic base region, a first extrinsic baseregion, a second extrinsic base region, an emitter on the intrinsic baselayer and a spacer adjacent the emitter and on the first extrinsic baseregion. The first extrinsic base region is adjacent the intrinsic baseregion and the second extrinsic base region is adjacent the firstextrinsic base region on the substrate, wherein a dopant concentrationof the second extrinsic base region is higher than a dopantconcentration of the first extrinsic base region.

Other embodiments of the invention will appear to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples to be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

1-16. (canceled)
 17. A heterojunction bipolar transistor, saidheterojunction bipolar transistor comprising: a substrate having acollector therein; an intrinsic base region, a first extrinsic baseregion adjacent said intrinsic base region and a second extrinsic baseregion adjacent said first extrinsic base region on said substrate,wherein a dopant concentration of said second extrinsic base region ishigher than a dopant concentration of said first extrinsic base region;an emitter on said intrinsic base layer; and a spacer adjacent saidemitter and on said first extrinsic base region.
 18. The heterojunctionbipolar transistor according to claim 17, wherein said intrinsic baseregion, said first extrinsic base region and said second extrinsic baseregion comprise SiGe base regions.
 19. The heterojunction bipolartransistor according to claim 17 further comprising an in situ steamgeneration oxide layer between said spacer and said first extrinsic baseregion.
 20. The heterojunction bipolar transistor according to claim 17further comprising a linear oxide layer and an in situ steam generationoxide layer between said first extrinsic base region and said spacer.21. The heterojunction bipolar transistor according to claim 17, whereinsaid spacer comprises a SiN spacer.